1. Field of the Invention
The present invention relates to active pixel sensors. More particularly, the present invention relates to variable biasing of the readout transistor in an active pixel sensor to improve sensitivity, reduce noise and to provide compressive non-linearity in the charge-to-voltage gain.
2. The Prior Art
In the art of CMOS active pixel sensors, the sensitivity, noise, and nature of the gain of an active pixel sensor present issues of concern. The sensitivity of an active pixel sensor in measuring the charge generated by the photons striking the active pixel sensor is typically characterized by determining the volts generated per photon of light striking the active pixel sensor and is termed charge-to-voltage gain. The readout amplifier in an active pixel sensor represents a substantial source of noise that in prior art pixel sensors has required design tradeoffs. The gain in prior art active pixel sensors is most often expansive, though it is preferred to be compressive.
The sensitivity of an active pixel sensor is determined by at least three factors. The first factor is related to the percentage of the area in the active pixel sensor available for converting photons to electrons. This is known as the fill factor. An increase in the area leads to an increase in the amount of charge generated. A second factor affecting the sensitivity of the active pixel sensor is related to the capacitance that is available for the integration of the charge sensed by the active pixel sensor. It will be appreciated that the voltage on the capacitor for given amount of charge is inversely proportional to the size of the capacitor. Accordingly, when the capacitance increases, the voltage decreases for the same amount of charge. A third factor is the gain of the readout amplifier for the active pixel sensor. Since the readout amplifier in the prior art is typically a transistor configured as a source follower, the gain is less than one.
One source of noise in an active pixel sensor is created by threshold fluctuations in the readout transistor. The amount of threshold fluctuation is related to the size of the readout transistor. As the size of the readout transistor is increased, the amount of threshold fluctuation, and hence the amount of noise decreases.
In compressive nonlinear gain, the gain at high light levels is less than the gain at low light levels. Those of ordinary skill in the art will appreciate that it is typically desirable to have greater sensitivity in converting photons-to-voltage at lower rather than higher light levels, because this increases the signal-to-noise ratio at lower light levels and, accordingly, the usable dynamic range of the active pixel sensor is increased.
The CMOS active pixel sensor art includes active pixel sensors that may or may not have embedded storage. FIGS. 1A and 2A illustrate typical CMOS active pixel sensors without and with embedded storage, respectively.
In an active pixel sensor 10 of FIG. 1A, a photodiode 12 employed to collect charge has an anode connected to a fixed voltage potential, shown as ground, and a cathode connected to the source of an N-channel MOS reset transistor 14 and the gate of an N-channel MOS readout transistor 16. The gate of N-channel MOS reset transistor 14 is connected to a RESET line, and the drain of N-channel MOS reset transistor 14 is connected to a voltage reference, Vref. The drain of N-channel MOS readout transistor 16 is connected to a fixed potential Vcc, and the drain of N-channel MOS readout transistor 16 is connected to an N-channel MOS row select transistor 18. Typically, the voltage Vref and the voltage Vcc are the same. In the active pixel sensor 10, the capacitance available for the integration of the charge sensed by the active pixel sensor 10 includes the photodiode 12 capacitance and the gate capacitance of the N-channel MOS readout transistor 16.
The operation of the active pixel sensor 10 as it is typically performed is well understood by those of ordinary skill in the art. A timing diagram corresponding to the operation of active pixel sensor 10 is depicted FIG. 1B. The active pixel sensor 10 is first reset by a RESET signal, during a reset step, that turns on N-channel MOS reset transistor 14 to place the voltage Vref on the cathode of the photodiode 12. An integration step begins when the RESET signal makes a transition from HIGH to LOW wherein photo-generated electrons are collected on the photodiode 12 to reduce the voltage on the cathode of the photodiode 12 from the value Vref placed there during the reset step. During a readout step, a ROW SELECT signal will be asserted to turn on N-channel MOS select transistor 18 to place the voltage at the source of N-channel MOS readout transistor 16 on the column output line for detection. It should be appreciated that the voltage on the gate of N-channel MOS readout transistor 16 formed by the charge accumulated on the cathode of the photodiode 12 will be followed by the source of N-channel MOS readout transistor 16.
In FIG. 2A, the CMOS active pixel sensor 30 has embedded storage. The active pixel sensor 30 includes a photodiode 32 having an anode that is connected to ground and a cathode that is connected to the source of N-channel MOS reset transistor 34. The gate of N-channel MOS reset transistor is connected to a RESET line, and the drain of N-channel MOS reset transistor 34 is connected to a voltage Vref. The cathode of photodiode 32 is also connected to the source of N-channel MOS transfer transistor 36. The gate of N-channel MOS transfer transistor 36 is connected to a XFR line, and the drain of N-channel MOS transfer transistor 36 is connected to a first plate of a capacitor 38 and to the gate of N-channel MOS readout transistor 40. The drain of N-channel MOS readout transistor 40 is connected to Vcc, and the source of N-channel MOS readout transistor 40 is connected to N-channel MOS select transistor 42. Typically, the voltage Vref and the voltage Vcc are equal to one another.
In the active pixel sensor 30, the capacitance available for the integration of the charge sensed by the active pixel sensor 30 includes the capacitance of a photodiode 32, the capacitance of the storage capacitor 38, and the gate capacitance of the N-Channel MOS readout transistor 40. It should be appreciated, however, that because the voltage at the drain of the N-channel MOS readout transistor 40 is high, the capacitance at the gate of the N-channel MOS readout transistor 40 is small and the gate capacitance of the N-channel MOS readout transistor 40 is not typically a preferred charge storage element.
A timing diagram corresponding to the operation of active pixel sensor 30 is depicted FIG. 2B. In the operation of the active pixel sensor 30, with the N-channel MOS transistor 34 turned on by a RESET signal to place the voltage Vref at the cathode of the photodiode 32, the N-channel MOS transfer transistor is also turned on by a signal asserted on the XFR line. When the N-channel MOS reset transistor 34 is turned off, the integration of photons striking the photodiode 32 begins. Since the N-channel MOS transfer transistor 36 is turned on, the capacitor 38 adds to the capacitance of the photodiode 32 during integration to increase the charge capacity and therefore, the intensity range of the storage pixel sensor 30. At the end of the integration period, the N-channel MOS transfer transistor 36 is turned off and the N-channel MOS row select transistor 42 is subsequently turned on so that the voltage at the gate of the N-channel MOS readout transistor 40 will be followed by the source of N-channel MOS readout transistor 40 to be placed on the column output.
In both active pixel sensors 10 and 30, by minimizing the gate area of the N-channel MOS readout transistors 16 and 40, respectively, the area provided to the photodiodes 12 and 32, respectively, can be made larger to improve the sensitivity by increasing the fill factor, and reducing the area for capacitance. Unfortunately, in reducing the gate capacitance of the N-channel MOS readout transistors 16 and 40, the noise in the N-channel MOS readout transistors 18 and 40 increases by an amount that is roughly in an inverse proportion to the gate areas of the N-channel MOS readout transistors 16 and 40. As such, when the gate area of the N-channel MOS readout transistors 16 and 40 is made smaller, the noise increases, and when the gate area of the N-channel MOS readout transistors 16 and 40 is made larger, the noise decreases.
In the case of the active pixel sensor 30, wherein the storage element 38 is included as a separate element, the sensitivity and noise issues are made more acute. The sensitivity is reduced because storage element 38 further reduces the fill factor, and the noise is increased because it reduces the available space for the N-channel MOS readout transistor 40.
Accordingly, it is an object of the present invention to increase the sensitivity of an active pixel sensor.
It is a further object of the present invention to decease the noise of an active pixel sensor.
It is yet another object of the present invention to compress the gain in an active pixel sensor as the relative light intensity increases.